The present invention relates to a battery charging circuit used to charge a battery.
Rechargeable batteries, such as lithium ion batteries, are often used in electronic devices. When charging such a battery with a charger, charging must be performed within a voltage range specified for the particular type of battery. A charging voltage that is less than the specified range will reduce the charge capacity of the battery. A charging voltage that is greater than the specified range will cause voltage stress to drastically decrease the charge capacity of the battery. Further, the charge current also has an upper limit. The charging voltage range for a lithium ion battery, for example, is narrow. Thus, if the input current from a charging power supply is restricted to an upper limit, the voltage is stabilized.
Japanese Laid-Open Patent Publication No. 8-237880 (page 1, FIG. 1) describes a charger in which input current from a charging power supply is stabilized by first and second current stabilization circuits, which respectively output a large current and a small current. A switching circuit first activates the first current stabilization circuit and charges a battery pack with the large current. As the battery voltage reaches a predetermined value and approaches a fully charged state, the switching circuit deactivates the first current stabilization circuit and activates the second current stabilization circuit to continue charging with a small current.
Japanese Laid-Open Patent Publication No. 9-233707 (page 1, FIG. 1) describes a charger that switches modes. In this charger, a switching circuit activates first and second current stabilization circuits and charges a battery pack with a large current. As the battery voltage reaches a predetermined value, the switching circuit deactivates the second current stabilization circuit to continue charging with just the activated first current stabilization circuit.
Referring to FIGS. 6A and 6B, a charging circuit for a lithium ion battery performs charging in two modes, namely, a trickle charge mode and a fast charge mode. The charging circuit enters a trickle charge mode when it starts charging. In the trickle charge mode, charging is performed by supplying current having a fixed and relatively small current value I1, as shown in FIG. 6B. This gradually increases the voltage of the battery.
As shown in FIG. 6A, when the voltage reaches a predetermined voltage value V1 (mode switching reference voltage), the charging circuit enters a second charging stage, namely, the fast charge mode. In the fast charge mode, charging is performed by supplying current having a fixed and relatively large current value I2.
When the voltage reaches a predetermined voltage value V2 in the fast charge mode, charging is continuously performed while maintaining the voltage value (voltage control mode). In this case, the charging current is gradually decreased. The charging ends when the charging current reaches a fixed current value I3.
A charging circuit 10 that performs such charging will now be discussed with reference to FIG. 4. The battery charging circuit 10 supplies charging current to a battery 50, which is connected to an external terminal TM1. The battery charging circuit 10 is supplied with voltage V10 via an external terminal TM2.
The external terminal TM2 is connected to the drain of an NMOS transistor 100. The source of the transistor 100 is connected to a resistor R0. Charging current is supplied to the battery 50 from the source of the transistor 100 via the external terminal TM1.
The gate of the transistor 100 is connected to the gate of another NMOS transistor 101. The drain of the transistor 101 is connected to the external terminal TM2 and supplied with the voltage V10. The transistors 100 and 101 form a current mirror circuit.
The source of the transistor 101 is connected to a switch 13. The switch 13 connects the source of the transistor 101 to either one of external terminals TM3 and TM4. A mode switching circuit 30 is connected to the switch 13. The mode switching circuit 30 measures the voltage between the two terminals of the battery 50 and provides the switch 13 with a switching signal for switching from the trickle charge mode to the fast charge mode when the voltage reaches the mode switching reference voltage.
When receiving the switching signal, the switch 13 changes connections from the external terminal TM3 to the external terminal TM4.
The external terminals TM3 and TM4 are also connected to a switch circuit 14. The switch circuit 14 is provided with the switching signal from the mode switching circuit 30 and switches connections in synchronism with the switch 13. In this manner, when provided with the switching signal, the switch 13 and switch circuit 14 each change connections from the external terminal TM3 to the external terminal TM4.
The external terminal TM3 is connected to a resistor R1, and the external terminal TM4 is connected to a resistor R2. The resistor R1 is used to determine the current value in the trickle charge mode. The resistor R2 is used to determine the current value in the fast charge mode. Generally, the charging current in the fast charge mode is set to be about twenty times greater than the charging current in the trickle charge mode. Accordingly, the resistance values of the two resistors R1 and R2 are also set to have a difference of about twenty times. Parasitic capacitances C1 and C2 are added to the resistors R1 and R2.
The switch circuit 14 has an output terminal connected to a non-inverting input terminal of an error amplifier 121. The error amplifier 121 is supplied with the voltage of the external terminal TM3 or TM4 that serves as an output voltage V14 of the switch circuit 14. Further, the error amplifier 121 has an inverting input terminal supplied with voltage V12, which serves as a current restriction reference voltage. The voltage V12 is used as a reference for restricting current. The output of the error amplifier 121 is provided to a mixer 120. The mixer 120 restricts the gate voltage supplied to the transistors 100 and 101 when the input voltage exceeds the reference value in the error amplifier 121 or an error amplifier 122.
The mixer 120 is also provided with the output of the error amplifier 122. The error amplifier 122 has an inverting input terminal supplied with voltage V13, which serves as a voltage restriction reference voltage, and a non-inverting input terminal, which is supplied with a divisional voltage produced by the resistor R0. The voltage V13 is used as a reference for restricting voltage.
The mixer 120 outputs voltage V11, which is supplied to the gates of the transistors 100 and 101.
In the battery charging circuit 10, when switching from the trickle charge mode to the fast charge mode, the connected one of the resistors R1 and R2 is changed. When the modes are switched in this manner, the current flowing through the resistor R2 is initially zero. Thus, as shown in FIG. 5A, the output voltage V14 of the switch circuit 14 temporarily falls immediately after switching from the resistor R1 to the resistor R2. To compensate for the fall in the output voltage V14, the charging current overshoots as shown in FIG. 5B. The fluctuation in the output voltage of the switch circuit 14 shown in FIG. 5A and the fluctuation in the current of FIG. 5B are enlarged with respect to the time axis and respectively shown in the graphs of FIGS. 5C and 5D. When such an overshooting occurs, the battery 50 is supplied with a charging current that exceeds the normal current value for the fast charge mode. Such a situation is not preferable for the battery 50.